The interaction-region design for a linac-ring electron-proton collider based on the LHC ('LR-LHeC') poses numerous challenges related to collision scheme, synchrotron radiation, aperture, magnet technology, and optics. We report a first assessment and various options

/Brookhaven

/CERN

/DESY

/INFN, Pisa /INFN, Bologna /DESY /SLAC

Aksakal, Husnu

Appleby, Robert

Bettoni, Simona

Bruning, Oliver

Chattopadhyay, Swapan

Ciftci, Abbas

Ciftci, Rena

Dainton, John

Eroglu, Emre

Holzer, Bernhard

Klein, Max

Korostelev, Maxim

Kostka, Peter

Litvinenko, Vladimir

Paoloni, Eugenio

Russenschuck, Stephan

Schulte, Daniel

Tapan, Ilhan

Tech., /Cockcroft Inst. Accel. Sci.

Tomas, Rogelio

U., /Ankara

U., /Liverpool

U., /Manchester

U., /Nigde

U., /Uludag

Zengin, Kahraman

Zimmermann, Frank

UNT Digital Library

INTERACTION-REGION DESIGN OPTIONS FOR A LINAC-RING LHECF. Zimmermann, R. Tomas, S. Bettoni, O. Brüning, B. Holzer, S. Russenschuck, D. Schulte,CERN, Switzerland; M. Sullivan, SLAC, U.S.A; A.K. Ciftci, R. Ciftci, K. Zengin, Ankara U.,Turkey; H. Aksakal, Nigde U., Turkey; E. Eroglu, I. Tapan, Uludag U., Turkey; R. Appleby,S. Chattopadhyay, M. Korostelev, Cockcroft Institute, UK; A. Polini, INFN Bologna, Italy;P. Kostka, U. Schneekloth, DESY, Germany; E. Paoloni, INFN Pisa, Italy;J. Dainton, M. Klein, Liverpool U., UK; V. Litvinenko, BNL, U.S.A.AbstractThe interaction-region design for a linac-ring electron-proton collider based on the LHC (“LR-LHeC”) [1, 2]poses numerous challenges related to collision scheme,synchrotron radiation, aperture, magnet technology, andoptics. We report a first assessment and various options.INTRODUCTIONWe shall collide an electron (positron) beam of 60-GeVenergy with the LHC 7-TeV proton beam [3]. At the inter-action point (IP) the electron beam size is matched to theproton beam size, σ∗p = σ∗e . We assume the LHC designvalues for the proton rms normalized transverse emittanceand bunch length. The proton IP beta function is taken tobe β∗x,y = 0.1 m [4, 5]. Table 1 lists other key parameters.The disruption angle θ0. is a conservative upper bound forthe largest deflection angle experienced during the colli-sion [6]. Its numerical value for the electrons equals about10 times the rms divergence of a non-colliding beam.Table 1: IP beam parameters of protons and electrons.protons electronsenergy [GeV] 7000 60Lorentz factor γ 7460 117400tr. norm. emittance γεx,y [μm] 3.75 50tr. geom. emittance εx,y [nm] 0.50 0.43IP beta function β∗x,y [m] 0.10 0.12rms IP beam size σ∗x,y [μm] 7 7rms IP divergence σ ′∗x,y [μrad] 70 58disruption parameter D 2× 10−6 6.0disruption angle θ0 [μrad] 0.06 572beam current [mA] 430–580 6.6HOURGLASS AND CROSSING ANGLEThe rms electron (positron) bunch length of 300 μm issmall compared with the proton bunch length of 7.5 cm.Approximating the longitudinal electron bunch distributionby a delta function, and assuming σ∗θc  σz,p, the geo-metric luminosity loss factor Hhg, describing the effect ofthe “hourglass” and of the finite (full) crossing angle θ c, isHhg =√πzez2erfc(z)/S, where (1)z ≡ 2(β∗e/σz,p)(εe/εp)√1 + (εe/εp)2S and S ≡√1 +σ2z,pθ2c8σ∗ 2.The dependence of the geometric loss factor on the cross-ing angle is illustrated in Fig. 1, for the parameters of Table1. The geometric loss factor of 0.91 for zero crossing angle(θc = 0) arises due to the long proton bunch length andthe small IP beta functions β∗x,y . The factor Hhg decreasesquickly for increasing θc values. A head-on collision is op-timum, and any residual crossing angle should not exceed150 μrad (about 2σ′∗x,y) where Hhg ≈ 0.8.0.5 1.0 1.5 2.00.20.40.60.81.00.0Hhgc[mrad]Figure 1: Geometric loss factor Hhg vs. crossing angle.A collision with zero crossing angle can be realizedeither (1) by inserting dipoles between the proton finalquadrupole triplets and the IP for aligning the two collid-ing beams, or (2) by tilting the proton bunches using crabcavities so as to yield an effective head-on collision whilethe bunch centroids intersect at a large angle.CONCEPTUAL LAYOUTThe LHeC detector encompasses a 3.5 T solenoid ex-tending ±6 m from the interaction point (IP) [7]. The freelength, l∗, between the end of the last quadrupole and theIP is taken to be 10 m, so that the first proton quadrupolemagnet on either side is located 4 m beyond the end of thedetector. The most important region for the inner detec-tor spans ±1.5 m on either side of the IP [7]. A detectoracceptance down to 1◦ is desired.For a first-quadrupole half aperture of 26 mm anelectron-beam exit hole in the magnet iron yoke could startat 60 mm from the magnet center. With a smaller aperturethe hole could be moved closer to the axis, and vice versa.A cooled beam screen may be required since the chamberinside this hole is likely to be hit by synchrotron radiation(SR). A dipole field inside the exit hole could help to de-flect the e− beam further away from the rest of the triplet.The size of the hole should provide sufficient beam stayclear for both disrupted and non-colliding electron beams.SLAC-PUB-15101Work supported by US Department of Energy contract DE-AC02-76SF00515.SLAC National Accelerator Laboratory, Menlo Park, CA 94025Head-on collisions can be provided by a dipole field thatstretches from about 1.5 m to 9 m from the IP on either side,leaving 1 m free space between the end of the dipole andthe entrance of the proton triplet (for shielding) and a 3-mgap around the IP (for the inner detector). In order for the7.5-m long dipole to displace the 60-GeV electron beam atthe quadrupole entrance by 80 mm from the proton beamaxis, a dipole field of 0.45 T is required, implying signifi-cant SR. The IR configuration with separation dipoles andSR fan is illustrated in Fig. 2. From this figure it also isevident that the paths of both electron beam and SR willmake it difficult to provide a full φ coverage and forward-backward symmetry of the detector acceptance down toθ = 1◦. Figure 2 also indicates the computed minimumstay clear corresponding to beam envelopes of 10σ for theelectrons and 11σ for the protons. with an additional con-stant radial margin of 10 mm for orbit errors, misalign-ments, optics errors etc.For a proton-bunch spacing of 25 ns the first parasiticcollision occurs at a distance s of 3.75 m from the IP.The 0.45-T dipole field produces a transverse separationof more than 50σ at this location.-10 -5 0 5 10-10-50510152025s [m]x [cm]shieldedcoile-p1o2o0.5oFigure 2: Beam envelopes of 10σ (electrons) [solid blue]or 11σ (protons) [solid green], the same envelopes withan additional constant margin of 10 mm [dashed], thesynchrotron-radiation fan [orange], and the approximatelocation of the magnet coil between incoming protons andoutgoing electron beam [black].An alternative approach for realizing head-on collisionis via crab cavities. It has the merit of avoiding any SRsources near the IP. Considering the same quadrupole ge-ometry as above, a crossing angle of 8 mrad is required.This crossing angle is about 25 times larger than thosefor the regular LHC pp collisions and for the LHC high-luminosity upgrades (SLHC). It would require a crab cav-ity voltage above 100 MV at 400 MHz RF frequency [8], tobe compared with typical crab-cavity voltages of 2–5 MVneeded for various SLHC scenarios. Thus this paper fo-cuses on the dipole-based collision scheme.SYNCHROTRON RADIATIONSR power from both the downstream and upstream bendmagnets is deposited on the face of the proton triplet. Witha deflection angle of 34 mrad on either side of the IP, thenumber of photons emitted per bunch passage is 8× 10 10.Their critical energy of 1.07 MeV approximately equals thethreshold for pair creation. For 6.6-mA beam current theaverage SR power from the dipole magnets is 87 kW. Morethan one quarter of these photons, corresponding to about26 kW of power, mainly from the downstream bend fan,will hit the shield protecting the magnet coil (Fig. 2), oneeighth (11 kW) will enter the proton beam pipe inside thefirst quadrupole Q1, and more than half of the total (50kW) will pass into the electron-beam exit channel. Figure3 (left) illustrates the horizontal distribution of photons atthe entrance face of the first proton quadrupole.The peak SR power density at the absorber shield isabout 1000 W/mm2 (Fig. 3 right). The surface of themagnet-coil protection shield should be sloped to reducethe power density and to make it acceptable for ordinarymaterials like copper. An acceptable number is 10-20W/mm2, the higher value referring to GlidCop (a disper-sion strengthened copper alloy). The SR absorbers have tobe water cooled. They could further be tailored to mini-mize the scattering of photons into the proton triplet beampipe. Another proposal is installing X-ray mirrors with ex-tremely small grazing angle to deflect part of the SR awayfrom the proton magnets [10]. The SR power entering theelectron beam pipe strikes at a shallow angle so that thepower density is not an issue.The upstream bend magnet deposits significant SRpower on any central beam pipe in the horizontal plane.One possible solution is to open up the inner-detector beampipe and let the radiation strike a beam-pipe surface out-side of the detector region [9]. The inner-detector beampipe could be elliptical with horizontal and vertical halfapertures of 6 cm (Fig. 2) and 2.5 cm, respectively, as-suming an off-center collision point and that the pipe canhold the vacuum pressure. This option removes a poten-tial source of background for the detector. However, if thebeam pipe grows then so does the downstream dipole mag-net, which reduces the detector acceptance. An alternativeapproach would be to have the SR hit the inner pipe [9].The SR surface density power would be reasonable for apipe with a few-cm radius. However, the pipe would haveto be quite thick and shielded in order to keep the detectorbackgrounds acceptably low. A future study of these var-ious options should include a simulation of the SR impactonto the beam pipe, and the associated magnet designs.0 50 100 150 200 250 30000.51.01.52.02.59photons/e- bunch/ x [10 /mm]x [mm]second bendfirst bendshieldforcoil0 50 100 150 200 250 300105010050010005000104power density [W/mm ]2x [mm]second bendfirst bendshieldforcoilFigure 3: Line density of photons hitting the entrance planeof the final quadrupoles per electron bunch passage (N e =2×109) and peak power density (Ie = 6.6 mA) at s = 10 mfrom the IP as a function of horizontal position.OPTICSThe proton IP beta function is a crucial parameter for theLHeC luminosity [3]. It is constrained by the aperture inthe triplet and by the chromatic correction. The nominalβ∗ value for LHC pp collisions is 0.55 m. For LHeC wetarget a smaller value of 0.1 m, which, according to scal-ing laws [11], should be achievable by a combination ofthree ingredients: (1) A shorter free length to the interac-tion point, l∗, of 10 m, instead of 23 m for the LHC ppcollisions, eases the requirements on the magnet aperture(∝ l∗) and reduces the chromaticity (∝ l∗/β∗). (2) Thetriplet aperture must accommodate only one squeezed pro-ton beam, instead of two for pp collisions, which increasesthe aperture available for the single main beam by some50%. By itself this would allow decreasing β∗ by morethan a factor of 2 aperture-wise. (3) Changing the super-conductor material from Nb-Ti to Nb3Sn may increase themaximum field and/or aperture by up to a factor of 2 [11].Since (1) and (2) together can already achieve β ∗ ≈ 0.1 m,the new superconductor is not strictly necessary for reach-ing β∗ = 0.1 m, but it provides additional safety margin,e.g. for a thicker beam screen and cold bore or for spuriousdispersion.If the LHC IPs 1 and 5 are squeezed to their nomi-nal value of β∗ = 0.55 m, the unused sextupole strengtharound the ring will suffice to correct the LHeC final-focuschromaticity for β∗ = 0.1 m, possibly after adjusting phaseadvances between arcs. If the other IPs are also squeezedto sub-nominal β∗ values, additional measures may be nec-essary, for example, introducing a large beta beat in the twoadjacent arcs [12] or a local chromatic correction.Figure 4 (left) shows the formal solution of squeezingthe present IR8 optics, with l∗ of 23 m, to β∗ = 0.1 m byvarying the matching quadrupoles Q4 to QT11, with all butone staying within their present strength limits. The tripletgradient is kept the same as for the nominal LHC. Aperturehas not been included as constraint. Figure 4 (right top)presents a proposed new proton optics with β ∗ = 0.1 mand l∗ = 10 m. For the electron beam the distance l∗ is nota critical parameter. We have chosen l∗ = 20 m (Fig. 4).D [m]x,y[km]s [m]024680 20 40 60 80 100 120 140 160s [m]e- β* = 0.10mβxβy0246810120 20 40 60 80 100 120 140 160proton β* = 0.10mβxβyprotonselectrons [km]Figure 4: LHC proton interaction-region optics for β ∗x,y =0.1 m, scaled from the nominal IR optics (left) [5], and anew IR optics with β∗x,y = 0.1 m for protons [l∗ = 10 m](top right) and electrons [l∗ = 20 m] (bottom right) [4].MAGNETSTable 2 lists parameters of the first two quadrupoles inthe final triplets for the proton beam, corresponding to theoptics in Fig. 4 (right). Magnets with the desired gradientsand aperture could be realized with Nb3Sn superconduc-tor, including an exit hole for the electron beam. Nb 3Snquadrupoles will be available by 2014 [13]. The electrontriplet magnets pose no particular challenge [4].Table 2: Parameters of the first two proton quadrupoles [4].magnet pipe radius gradient field at pipeQ1 26 mm 318.6 T/m 8.4 TQ2 36 mm 250.0 T/m 9.1 TOne option for implementing the dipole field would bea permanent magnet made from SmCo, as in PEP-II [9].The SR could be extracted with a proper opening in thedipole or else it will hit the beam pipe inside the magnet.Effects of exposure to SR and collision debris will need tobe looked at. Another option for generating the dipole fieldwould be via a pair of helical coils [14]. For a 0.4-T field,these coils could be either normal- or superconducting. Asingle tilted helical coil rather than a pair might suffice,and the solenoid part of the field could conceivably be usedto partially compensate part of the detector solenoid. Asimilar field could also be obtained by deforming the coilsof the detector solenoid, like the detector-integrated dipoleof the ILC [15], which would be a third option.SECOND PROTON BEAMA second LHC proton beam moving in opposite direc-tion may need to be transported across the LHeC interac-tion region. If this second proton beam shares a commonIR vacuum chamber with the main proton beam the physi-cal aperture available for either beam will be reduced. Thisloss in aperture can be partially mitigated by not squeez-ing the second beam, so that the latter would have a muchsmaller size inside the final quadrupole triplet. However,in this case, the inner detector beam pipe might need to beopened up for the second proton beam.Another possibility would be designing the LHeC detec-tor with a bypass hole for the second proton beam. An eas-ily achievable transverse separation would be 0.4–0.5 m.The triplet magnet cryostat for the colliding proton beammust then also feature a hole for the second proton beamwith 2–3 cm radius at about 50 cm from its axis.REFERENCES[1] F. Zimmermann et al, Proc. EPAC’08 Genoa, 2847 (2008).[2] F. Zimmermann et al, FR1PBO5, PAC’09 Vancouver,[3] F. Zimmermann et al, “RLA and ERL Designs for a Linac-Ring LHeC,” this conference.[4] R. Tomas, “IR Design for LR,” in [7].[5] M. Korostelev, private communication (2010).[6] P Chen, K. Yokoya, Physical Review D38, 987 (1988).[7] 2nd ECFA-CERN LHeC Workshop, Divonne, Sept. 2009.[8] F. Zimmermann, U. Dorda, Proc. HHH-LUMI-05, Arci-dosso, CERN-2006-008, p. 67 [Table 4] (2005).[9] M. Sullivan, “Notes & Thoughts about LHeC,” 26.03.2010.[10] A.K. Ciftci et al, “Deflecting Synchrotron Radiation fromthe IR of a Linac-Ring LHeC,” this conference.[11] E. Todesco, J.-P. Koutchouk, Proc. HHH LHC-LUMI-06,Valencia, CERN Report CERN 2007-002, p. 61 (2007)[12] S. Fartoukh, private communication, 22 March 2010.[13] G-L. Sabbi, US-LARP CM14, 26 April 2010.[14] S. Ishmael et al, IEEE Tr. Appl. SC, Vol. 18, 2, 693 (2008).[15] R.P. Smith et al, IEEE Tr. Appl. SC, Vol. 16, 2, 489 (2006).

Interaction-Region Design Options for a Linac-Ring LHeC

Interaction-Region Design Options for a Linac-Ring LHeC

Abstract

Similar works

Full text

Available Versions

UNT Digital Library